JP2013095928A - High tensile strength steel sheet excellent in toughness and manufacturing method thereof - Google Patents
High tensile strength steel sheet excellent in toughness and manufacturing method thereof Download PDFInfo
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本発明は、靭性に優れた高張力鋼板とその製造方法に関する。より詳しくは、高度の安全性が要求される海洋構造物や低温ガス貯蔵タンクなど大形鉄鋼構造物用途に好適な、降伏強度４００ＭＰａ以上、板厚４０ｍｍ以上の高張力鋼板及びその製造方法に関する。 The present invention relates to a high-tensile steel plate having excellent toughness and a method for producing the same. More specifically, the present invention relates to a high-tensile steel plate having a yield strength of 400 MPa or more and a plate thickness of 40 mm or more, and a method for producing the same, suitable for large steel structures such as offshore structures and low-temperature gas storage tanks that require a high degree of safety.
近年エネルギー需要が益々増加の傾向にあり、海底石油資源の探索が活発化している。これらに使用される構造物は例えば、プラットフォーム、ジャッキアップリグのように大型化しており、これに伴い使用鋼材が厚肉化し、より安全性の確保が重要な課題である。 In recent years, energy demand has been increasing, and search for offshore oil resources has become active. The structures used for these are, for example, larger in size such as platforms and jack-up rigs, and as a result, the steel materials used become thicker, and ensuring safety is an important issue.
前記のような大型構造物では降伏強度４００ＭＰａ以上の高強度の鋼材が用いられ、その板厚も１００ｍｍを超える極厚高張力鋼が用いられることがある。 In such a large structure, a high-strength steel material having a yield strength of 400 MPa or more is used, and an ultra-thick high-tensile steel having a plate thickness exceeding 100 mm may be used.
また、海底石油資源の探索地域が近年寒冷地や大水深域へと移っており、それらの海域で稼動する海洋構造物は極めて厳しい気象・海洋条件に晒される。このため、これらの鋼材には、例えば−４０℃以下という非常に厳しい低温域での靭性が要求されると共に、溶接性も当然要求される。 In addition, search areas for submarine oil resources have recently moved to cold regions and deep waters, and offshore structures operating in those regions are exposed to extremely severe weather and ocean conditions. For this reason, these steel materials are required to have toughness in a very severe low temperature range of, for example, −40 ° C. or lower, and of course weldability.
さらに、安全性の面からもユーザーの検査基準は厳しく、母材、溶接部ともに従来のシャルピー衝撃値の規定に加え、最低使用温度におけるＣＴＯＤ（Crack tip Opening displacement：き裂先端開口変位）値も規定されるようになってきている。すなわち、１０ｍｍ各に切断採取する微小評価試験であるシャルピー試験で安定した特性を得た場合であっても、全厚のＣＴＯＤ試験では特性を満足できない場合が多く発生している。 In addition, the inspection standards for users are strict from the viewpoint of safety. In addition to the conventional Charpy impact values for both the base metal and welded parts, the CTOD (Crack tip Opening displacement) value at the minimum operating temperature is also included. It is becoming regulated. That is, even when stable characteristics are obtained by the Charpy test, which is a micro evaluation test cut and sampled at 10 mm intervals, there are many cases where the characteristics cannot be satisfied by the full thickness CTOD test.
例えば、特許文献１には、低温での溶接部靱性に優れた高張力鋼に係る発明が開示されている。特許文献１に記載の発明は、３００ｋＪ／ｃｍ以上の大入熱溶接が可能な、溶接性に優れた降伏応力４２０Ｎ／ｍｍ２以上の高張力鋼に主眼を置いており、Ｎ／Ａｌ比をコントロールすると共にある程度粗大化させたＣｕ粒子を均一に分散させることを特徴とする。しかしながら、全厚のＣＴＯＤ特性は特性を満足できない場合がある。 For example, Patent Document 1 discloses an invention related to high-tensile steel excellent in weld zone toughness at low temperatures. The invention described in Patent Document 1 focuses on high-tensile steel with yield stress of 420 N / mm 2 or more, which is capable of high heat input welding of 300 kJ / cm or more and has excellent weldability, and has an N / Al ratio of It is characterized by uniformly dispersing Cu particles which are controlled and coarsened to some extent. However, the full thickness CTOD characteristic may not satisfy the characteristic.
本発明は、上記現状に鑑みてなされたもので、その目的は、高度の安全性が要求される海洋構造物などの大形鉄鋼構造物用途に好適な、降伏強度４００ＭＰａ以上、ＣＴＯＤ値０．３ｍｍ以上、板厚４０ｍｍ以上の靭性に優れた高張力鋼板およびその製造方法を提供することである。 The present invention has been made in view of the above-mentioned present situation, and its object is to provide a yield strength of 400 MPa or more, a CTOD value of 0. 0, which is suitable for use in large steel structures such as offshore structures that require high safety. It is to provide a high-tensile steel plate excellent in toughness of 3 mm or more and a plate thickness of 40 mm or more and a method for producing the same.
本発明者らは、海洋構造物に適した溶接性と靱性に優れた厚肉高強度鋼板を開発することを目的に、鋼成分およびその製造方法について種々の実験を行なった結果、以下の(a)〜(c)に示す知見を得た。 The present inventors conducted various experiments on steel components and production methods for the purpose of developing a thick high-strength steel plate excellent in weldability and toughness suitable for offshore structures. The findings shown in a) to (c) were obtained.
(a) 化学成分の調整
全厚のＣＴＯＤ特性を安定化させるには、添加合金成分の調整が重要である。特に、海洋構造物用鋼材においては、極小入熱時のＨＡＺ硬化が課題となる。これを解消するため、Ｃ、Ｍｎ、Ｃｒ、Ｍｏ、Ｃｕ、Ｎｉ等の添加量調整により焼入性を最適化することが重要であるが、特に、Ｎｂ、Ｂの低減が重要であることを明らかにした。Ｎｂは制御圧延時の細粒化に有効な元素であり、Ｂは大入熱溶接時の溶接熱影響部靭性の特性安定化に有効であるが、極小入熱時の特性安定化のためには、積極添加を避けるのみならず、それぞれを不純物としても極力低減することが重要である。
(a) Adjustment of chemical composition In order to stabilize the CTOD characteristics of the full thickness, it is important to adjust the additive alloy composition. In particular, in steel materials for offshore structures, HAZ hardening at the minimum heat input becomes a problem. In order to eliminate this, it is important to optimize the hardenability by adjusting the amount of addition of C, Mn, Cr, Mo, Cu, Ni, etc. In particular, it is important to reduce Nb and B. Revealed. Nb is an element effective for fine graining during controlled rolling, and B is effective for stabilizing the properties of weld heat affected zone toughness during high heat input welding, but for stabilizing properties during minimal heat input. It is important not only to avoid positive addition, but also to reduce each of them as impurities as much as possible.
(b) 板厚中心部の組織、偏析、非金属在物の制御
全厚のＣＴＯＤ特性を安定化させるには、添加合金成分の調整のみでは不十分であり、板厚中心部における偏析、介在物の制御が不可欠である。これは、構造用鋼に一般的に適用される連続鋳造においては最終凝固部である板厚中心部に偏析および非金属介在物が形成され、これが破壊の起点になる多めに全厚のＣＴＯＤ特性が不安定となるためである。このため、鋼材の添加化学成分の調整に加え、板厚中心部における組織、Ｃ偏析、Ｍ−Ａ量、非金属在物の制御の制御が必要である。
(b) Control of the structure, segregation, and non-metallic inclusions in the center of the plate thickness In order to stabilize the CTOD characteristics of the total thickness, it is not sufficient to adjust the additive alloy components alone. Control of things is essential. This is because, in continuous casting generally applied to structural steel, segregation and non-metallic inclusions are formed at the center of the plate thickness, which is the final solidified part, and this is the starting point of fracture, so the CTOD characteristics of the full thickness This is because becomes unstable. For this reason, in addition to the adjustment of the chemical component added to the steel material, it is necessary to control the control of the structure, C segregation, MA amount, and non-metallic presence in the center of the plate thickness.
(c) 上記(a)および(b)を満たす鋼材において、４０ｍｍ以上の厚肉材で板厚中心部まで降伏強度が４００ＭＰａ以上の高強度と全厚のＣＴＯＤ特性を満足するには、鋼材の製造方法も重要となる。Ｎｂ、Ｂを含有しない成分系での高強度-高靭性確保には厳格な制御圧延が必要となる。すなわち、板厚中心部までできるだけ多くの微細フェライトを分散させた上で、強度確保のためにε−Ｃｕ粒子を時効析出させることで高強度化を図る手段が有効である。このために加熱圧延条件および水冷条件を厳格に制御する必要がある。 (c) In the steel material satisfying the above (a) and (b), in order to satisfy the CTOD characteristics of the steel plate satisfying the high strength with a yield strength of 400 MPa or more and the total thickness up to the center of the plate thickness with a thick material of 40 mm or more, The manufacturing method is also important. Strict controlled rolling is required to ensure high strength and high toughness in a component system that does not contain Nb and B. That is, it is effective to increase the strength by dispersing as much fine ferrite as possible to the center of the plate thickness and then age-depositing ε-Cu particles to ensure the strength. For this reason, it is necessary to strictly control the heating and rolling conditions and the water cooling conditions.
本発明は、上記の知見に基づいて完成されたものであり、その要旨は、下記(1)〜(5)に示す高張力鋼板及び(6)に示す高張力鋼板の製造方法にある。 The present invention has been completed based on the above findings, and the gist thereof resides in the high-tensile steel plate shown in the following (1) to (5) and the method for producing the high-tensile steel plate shown in (6).
(1) 質量％で、Ｃ：０．０１〜０．０８％、Ｓｉ：０．０１〜０．３％、Ｍｎ：１．０〜２．０％、Ｐ：０．０１２％以下、Ｓ：０．００５％以下、Ｃｕ：０．８〜２．０％、Ｎｉ：０．１〜２．５％、Ｃｒ：０．０１〜０．５％、Ｔｉ：０．００５〜０．０３％、Ａｌ：０．００１〜０．０５０％及びＮ：０．００１〜０．０１％を含有し、残部はＦｅ及び不純物からなり、不純物中のＮｂ：０．００１％以下、Ｂ：０．０００３％以下、Ｏ：０．００３％以下である化学組成を有し、板厚中心部における結晶粒径２０μｍ以下のフェライト分率が６０％以上、板厚中心部における島状マルテンサイト組織の面積率が４．０％以下、板厚中心部における介在物量が、ＪＩＳ Ｇ ０５５５における点算法にて０．０２０％以下、板厚中心部におけるＣ含有量が０．１２％以下であることを特徴とする、板厚中心部の降伏強度が４００ＭＰａ以上の靭性に優れた高張力鋼板。 (1) By mass%, C: 0.01 to 0.08%, Si: 0.01 to 0.3%, Mn: 1.0 to 2.0%, P: 0.012% or less, S: 0.005% or less, Cu: 0.8-2.0%, Ni: 0.1-2.5%, Cr: 0.01-0.5%, Ti: 0.005-0.03%, Al: 0.001 to 0.050% and N: 0.001 to 0.01%, with the balance being Fe and impurities, Nb in impurities: 0.001% or less, B: 0.0003% Hereinafter, O: having a chemical composition of 0.003% or less, a ferrite fraction having a crystal grain size of 20 μm or less in the center portion of the plate thickness is 60% or more, and an area ratio of the island martensite structure in the center portion of the plate thickness is 4.0% or less, the amount of inclusions at the center of the plate thickness is 0.020% or less according to the point method in JIS G 0555, and at the center of the plate thickness. High-tensile steel sheet that C content is equal to or less than 0.12%, the yield strength of the center of plate thickness and excellent more toughness 400 MPa.
(2) Ｆｅの一部に代えて、質量％で、Ｍｏ：０．５％以下、Ｖ：０．０５％以下から選択される１種以上を含有することを特徴とする上記(1)の高張力鋼板。 (2) Instead of a part of Fe, by mass%, it contains at least one selected from Mo: 0.5% or less, V: 0.05% or less. High tensile steel plate.
(3) Ｆｅの一部に代えて、質量％で、Ｓｎ：０．５０％以下を含有することを特徴とする上記(1)または(2)の高張力鋼板。 (3) The high-tensile steel sheet according to (1) or (2) above, which contains Sn: 0.50% or less in mass% instead of part of Fe.
(4) Ｆｅの一部に代えて、質量％で、Ｃａ：０．００５％以下、Ｍｇ：０．００５％以下、ＲＥＭ：０．００５％以下から選択される１種以上を含有することを特徴とする上記(1)〜(3)のいずれかの高張力鋼板。 (4) Instead of a part of Fe, by mass%, it contains at least one selected from Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less. The high-tensile steel plate according to any one of (1) to (3) above.
(5) 上記(1)〜(4)のいずれかの化学組成を有するスラブを、９００〜１１８０℃の温度域に加熱し、７００〜８００℃の温度域で累積圧下率５０％以上の熱間圧延を施し、７００℃以上の温度で圧延を終了し、６５０℃以上の温度域から水冷を開始し、５００℃以下になるまで、板厚の１／４の位置における平均冷却速度５℃／ｓｅｃ以上で水冷し、その後、５５０〜６７０℃の温度域に再加熱することを特徴とする、板厚中心部の降伏強度が４００ＭＰａ以上の靱性に優れた高張力鋼板の製造方法。 (5) A slab having the chemical composition of any one of the above (1) to (4) is heated to a temperature range of 900 to 1180 ° C., and a hot reduction of 50% or more in the temperature range of 700 to 800 ° C. Rolling is completed, rolling is finished at a temperature of 700 ° C. or higher, water cooling is started from a temperature range of 650 ° C. or higher, and an average cooling rate at a position of 1/4 of the sheet thickness is 5 ° C./sec until the temperature becomes 500 ° C. or lower. A method for producing a high-tensile steel sheet excellent in toughness with a yield strength of 400 MPa or more, characterized in that it is cooled with water and then reheated to a temperature range of 550 to 670 ° C.
本発明によれば、降伏強度４００ＭＰａ以上、ＣＴＯＤ値０．３ｍｍ以上、板厚４０ｍｍ以上の靭性に優れた高張力鋼板およびその製造方法を提供することができる。 According to the present invention, it is possible to provide a high-tensile steel plate excellent in toughness having a yield strength of 400 MPa or more, a CTOD value of 0.3 mm or more, and a plate thickness of 40 mm or more, and a method for producing the same.
以下、本発明について詳細に説明する。まず、本発明を上記のような鋼成分に限定した理由を述べる。 Hereinafter, the present invention will be described in detail. First, the reason why the present invention is limited to the steel components as described above will be described.
（Ａ）鋼板の化学組成
Ｃ：０．０１〜０．０８％
Ｃは、鋼材の強度確保に有効な元素である一方、多すぎると島状マルテンサイトを生成して母材および溶接熱影響部の靭性を劣化させる。このため添加量を０．０１〜０．０８％とした。Ｃ含有量の好ましい下限は０．０２％であり、Ｃ含有量の好ましい上限は０．０５％である。
(A) Chemical composition of steel plate C: 0.01 to 0.08%
C is an element effective for ensuring the strength of the steel material. On the other hand, if it is too large, island-shaped martensite is generated and the toughness of the base material and the weld heat affected zone is deteriorated. For this reason, the addition amount was set to 0.01 to 0.08%. A preferable lower limit of the C content is 0.02%, and a preferable upper limit of the C content is 0.05%.
Ｓｉ：０．０１〜０．３％
Ｓｉは溶鋼の予備脱酸に有効な元素であるが、セメンタイト中に固溶しないため、多量に添加されると未変態オーステナイト粒がフェライト粒とセメンタイトに分解するのを阻害し、島状マルテンサイトの生成を助長する。これらの理由から、Ｓｉの添加量は、０．０１〜０．３％とする。Ｓｉ含有量の好ましい下限は０．０５％であり、Ｓｉ含有量の好ましい上限は０．２％である。
Si: 0.01 to 0.3%
Si is an effective element for preliminary deoxidation of molten steel, but since it does not dissolve in cementite, when added in a large amount, it inhibits the decomposition of untransformed austenite grains into ferrite grains and cementite, resulting in island martensite. Contributes to the generation of For these reasons, the addition amount of Si is set to 0.01 to 0.3%. A preferable lower limit of the Si content is 0.05%, and a preferable upper limit of the Si content is 0.2%.
Ｍｎ：１．０〜２．０％
Ｍｎは強度確保に必要な元素であるとともに、脱酸剤としても有効な元素である。しかし、Ｍｎの過剰な添加は、焼入れ性を過剰に増加させ溶接性およびＨＡＺ靱性を劣化させる。さらに、Ｍｎは中心偏析を助長する元素としてしられているので、中心偏析抑制の観点からはその含有量を抑制する必要がある。したがってＭｎの含有量は１．０〜２．０％とする。Ｍｎ含有量の好ましい下限は１．３％であり、Ｍｎ含有量の好ましい上限は１．８％である。
Mn: 1.0-2.0%
Mn is an element necessary for ensuring strength and is also an effective element as a deoxidizer. However, excessive addition of Mn excessively increases hardenability and degrades weldability and HAZ toughness. Furthermore, since Mn is an element that promotes center segregation, it is necessary to suppress its content from the viewpoint of suppressing center segregation. Therefore, the Mn content is set to 1.0 to 2.0%. The minimum with preferable Mn content is 1.3%, and the preferable upper limit of Mn content is 1.8%.
Ｐ：０．０１２％以下
Ｐは鋼に不可避的に含有される不純物元素であり、粒界偏析元素であるためにＨＡＺにおける粒界割れの原因となる。さらに母材靱性、溶接金属部とＨＡＺの靱性を向上させ、スラブ中心偏析も低減させるためには、その含有量は０．０１２％以下とする。好ましくは０．０１０％以下である。
P: 0.012% or less P is an impurity element inevitably contained in steel, and because it is a grain boundary segregation element, it causes grain boundary cracking in HAZ. Furthermore, in order to improve the toughness of the base metal, the weld metal part and the HAZ, and to reduce the slab center segregation, the content is made 0.012% or less. Preferably it is 0.010% or less.
Ｓ：０．００５％以下
Ｓは多量に存在する場合、溶接割れ起点となるMnＳ単体の析出物を生成し、特に最終凝固部である板厚中心部の清浄度を劣化させる。このため含有量を０．００５％以下とする。好ましくは０．００２％以下である。
S: 0.005% or less When S is present in a large amount, a precipitate of MnS as a starting point for weld cracking is generated, and in particular, the cleanliness of the central portion of the plate thickness, which is the final solidified portion, is deteriorated. For this reason, content is made into 0.005% or less. Preferably it is 0.002% or less.
Ｃｕ：０．８〜２．０％
Ｃｕは鋼材の強度および靱性を高める効果があるが、ＨＡＺ靱性に対する悪影響も少ない。特に、時効処理時のε−Ｃｕ析出による強度上昇効果を期待する上で０．８％以上必要である。しかし、Ｃｕ含有量が高くなると溶接高温割れ感受性が高くなり、予熱などの溶接施工が複雑になるため、その含有量は２．０％以下とする。Ｃｕ含有量の好ましい下限は０．９％であり、Ｃｕ含有量の好ましい上限は１．２％である。
Cu: 0.8 to 2.0%
Cu has an effect of increasing the strength and toughness of the steel material, but has little adverse effect on the HAZ toughness. In particular, 0.8% or more is necessary in order to expect the effect of increasing the strength due to ε-Cu precipitation during the aging treatment. However, if the Cu content is increased, the sensitivity to hot cracking of the weld is increased, and welding work such as preheating is complicated. Therefore, the content is set to 2.0% or less. A preferable lower limit of the Cu content is 0.9%, and a preferable upper limit of the Cu content is 1.2%.
Ｎｉ：０．１〜２．５％
ＮｉはＣｕと同様に鋼材の強度および靱性を高め、さらにＨＡＺ靱性を高めるための有効な元素であるが、過剰に添加してもコストアップに見合うだけの効果を得ることができないため、Ｎｉの含有量を ０．１〜２．５％とした。Ｎｉ含有量の好ましい下限は０．２％であり、Ｎｉ含有量の好ましい上限は１．５％である。
Ni: 0.1 to 2.5%
Ni is an effective element for increasing the strength and toughness of the steel material as well as Cu, and further increasing the HAZ toughness. The content was set to 0.1 to 2.5%. The preferable lower limit of the Ni content is 0.2%, and the preferable upper limit of the Ni content is 1.5%.
Ｃｒ：０．０１〜０．５％
Ｃｒは、鋼材の強度を高める効果があるが、過剰に添加すると母材靱性や溶接性の劣化を招く。したがって、含有量を０．０１〜０．５％とした。Ｃｒ含有量の好ましい下限は０．１％であり、Ｃｒ含有量の好ましい上限は０．３％である。
Cr: 0.01 to 0.5%
Cr has an effect of increasing the strength of the steel material, but if added excessively, the base material toughness and weldability are deteriorated. Therefore, the content is set to 0.01 to 0.5%. A preferable lower limit of the Cr content is 0.1%, and a preferable upper limit of the Cr content is 0.3%.
Ｔｉ：０．００５〜０．０３％
Ｔｉは、オーステナイト粒の微細化のために不可欠な元素である。また、連続鋳造鋳片の横ひび割れを防止する上でもその添加が不可欠である。しかし、過剰に添加すると母材靱性やＨＡＺの靱性が著しく損なわれる。したがって、Ｔｉの含有量を０．００５〜０．０３％とした。Ｔｉ含有量の好ましい下限は０．０１％であり、Ｔｉ含有量の好ましい上限は０．０１５％である。
Ti: 0.005 to 0.03%
Ti is an essential element for refining austenite grains. Moreover, the addition is indispensable also in preventing the lateral crack of a continuous cast slab. However, if added excessively, the toughness of the base metal and the toughness of HAZ are significantly impaired. Therefore, the content of Ti is set to 0.005 to 0.03%. A preferable lower limit of the Ti content is 0.01%, and a preferable upper limit of the Ti content is 0.015%.
Ａｌ：０．００１〜０．０５０％
Ａｌは溶鋼の予備脱酸に有効な元素であるが、過剰に添加すると島状マルテンサイトの生成を助長するため母材および溶接熱影響部の靭性が低下する。このため添加量を０．００１〜０．０５％とする。Ａｌ含有量の好ましい下限は０．０１％であり、Ａｌ含有量の好ましい上限は０．０３％である。
Al: 0.001 to 0.050%
Al is an element effective for preliminary deoxidation of molten steel, but if added excessively, the formation of island martensite is promoted, so the toughness of the base metal and the weld heat affected zone is lowered. For this reason, the addition amount is set to 0.001 to 0.05%. The preferable lower limit of the Al content is 0.01%, and the preferable upper limit of the Al content is 0.03%.
Ｎ：０．００１〜０．０１０％
Ｎは、窒化物を形成することで組織の細粒化に寄与するが、過剰に添加した場合には窒化物の凝集を通じて靭性を劣化させる。従って含有量を０．００１〜０．０１０％とする。Ｎ含有量の好ましい下限は０．００３％であり、Ｎ含有量の好ましい上限は０．００８％である。
N: 0.001 to 0.010%
N contributes to the refinement of the structure by forming nitrides, but when added excessively, N deteriorates toughness through aggregation of nitrides. Therefore, the content is made 0.001 to 0.010%. A preferable lower limit of the N content is 0.003%, and a preferable upper limit of the N content is 0.008%.
本発明に係る鋼板は、上記の成分を含有し、残部がＦｅおよび不純物からなる。ここで、不純物とは、鋼材を工業的に製造する際に鉱石やスクラップ等のような原料をはじめとして製造工程の種々の要因によって混入する成分であって、本発明に悪影響を与えない範囲で許容されるものを意味するものであるが、本発明においては、特に、不純物中のＮｂ、Ｂ及びＯ（酸素）について、次のとおりに規定する。 The steel sheet according to the present invention contains the above components, with the balance being Fe and impurities. Here, the impurities are components that are mixed due to various factors of the manufacturing process including raw materials such as ore and scrap when industrially manufacturing steel materials, and in a range that does not adversely affect the present invention. In the present invention, Nb, B, and O (oxygen) in the impurities are defined as follows.
Ｎｂ：０．００１％以下
Ｎｂは、オーステナイトの低温域で微細なＮｂ炭窒化物を形成することによりオーステナイト粒を微細化する作用を有する。更に、析出したＮｂ炭窒化物は圧延などによる加工を受けた未再結晶オーステナイト粒の回復、再結晶を抑制する効果を有しており、母材靱性の確保に有効である。しかしながら、極小入熱溶接におけるＣＴＯＤ特性を劣化させることから、本発明においては可能な限り含有量を低減させる。このため、Ｎｂ含有量を０．００１％以下とした。
Nb: 0.001% or less Nb has the effect of refining austenite grains by forming fine Nb carbonitrides in the low temperature range of austenite. Further, the precipitated Nb carbonitride has an effect of suppressing the recovery and recrystallization of unrecrystallized austenite grains that have been processed by rolling or the like, and is effective in securing the base material toughness. However, since the CTOD characteristic in the minimum heat input welding is deteriorated, the content is reduced as much as possible in the present invention. For this reason, Nb content was made into 0.001% or less.
Ｂ：０．０００３％以下
Ｂは、極微量で鋼材の強度を上昇させる元素であるが、極小入熱溶接におけるＣＴＯＤ特性を劣化させることから、本発明においては可能な限り含有量を低減させる。このため、Ｂ含有量を０．０００３％以下とした。
B: 0.0003% or less B is an element that increases the strength of the steel material in a very small amount, but degrades the CTOD characteristics in the minimum heat input welding, and therefore the content is reduced as much as possible in the present invention. For this reason, B content was made into 0.0003% or less.
Ｏ：０．００３％以下
Ｏ(酸素)は多量に存在すると清浄度の劣化が著しくなるため、母材、溶接金属部およびＨＡＺともに実用的な靱性確保が困難となる。そこで、０．００３％以下とする。好ましくは０．００２％以下である。
O: 0.003% or less When a large amount of O (oxygen) is present, the cleanliness deteriorates remarkably, making it difficult to ensure practical toughness for the base metal, the weld metal part, and the HAZ. Therefore, the content is made 0.003% or less. Preferably it is 0.002% or less.
本発明に係る鋼板には、必要に応じて、次の第１群から第３群までの少なくとも１群から選んだ成分の１種以上を含有させることができる。以下、これらの群に属する成分について述べる。 The steel plate according to the present invention may contain one or more components selected from at least one group from the following first group to third group, if necessary. Hereinafter, components belonging to these groups will be described.
第１群：Ｍｏ、Ｖ
Ｍｏ：０．５％以下
Ｍｏは、必要に応じて含有させることができる。Ｍｏを含有させると、鋼材の焼入れ性を増し、強度を向上させることができる。しかしながら、その含有量が０．５％を超えると、溶接金属部およびＨＡＺを硬化させ溶接低温割れ感受性を増大させるので、その含有量の上限は０．５％とする。好ましい上限は０．３％である。なお、Ｍｏによる強度向上効果を得たい場合には、Ｍｏを０．０３％以上含有させるのが望ましい。
First group: Mo, V
Mo: 0.5% or less Mo can be contained as necessary. When Mo is contained, the hardenability of the steel material can be increased and the strength can be improved. However, if the content exceeds 0.5%, the weld metal part and the HAZ are hardened and the weld cold cracking sensitivity is increased, so the upper limit of the content is 0.5%. A preferable upper limit is 0.3%. In addition, when obtaining the strength improvement effect by Mo, it is desirable to contain Mo 0.03% or more.
Ｖ：０．０５％以下
Ｖは、必要に応じて含有させることができる。Ｖを含有させると、Ｍｏと同様に、鋼材の焼入れ性を増し、強度を向上させることができる。しかしながら、その含有量が０．０５％を超えると、溶接金属部およびＨＡＺを硬化させ溶接低温割れ感受性を増大させるので、その含有量の上限は０．０５％とする。好ましい上限は０．０４％である。なお、Ｖによる強度向上効果を得たい場合には、Ｖを０．０１％以上含有させるのが望ましい。
V: 0.05% or less V can be contained as necessary. When V is contained, the hardenability of the steel material can be increased and the strength can be improved, similarly to Mo. However, if its content exceeds 0.05%, the weld metal part and HAZ are hardened and the weld cold cracking susceptibility is increased, so the upper limit of its content is 0.05%. A preferable upper limit is 0.04%. In addition, when obtaining the strength improvement effect by V, it is desirable to contain V 0.01% or more.
第２群：Ｓｎ
Ｓｎ：０．５０％以下
Ｓｎは、必要に応じて含有させることができる。Ｓｎを含有させると、Ｓｎ３＋となって溶解し、酸性塩化物溶液中でのインヒビター作用により腐食を抑制することができる。。また、Ｆｅ３＋を速やかに還元させ、酸化剤としてのＦｅ３＋濃度を低減することにより、Ｆｅ３＋の腐食促進作用を抑制するので、飛来塩分量が多い環境下での耐候性を向上させることができる。さらに、Ｓｎには鋼のアノード溶解反応を抑制するので、耐食性を向上させることもできる。しかし、その含有量が０．５０％を超えると脆化を起こす原因となるので、Ｓｎの含有量は０．５０％以下とする。好ましくは０．２０％以下である。なお、Ｓｎによるこれらの効果を得たい場合には、Ｓｎの含有量は０．０３％以上とするのが望ましい。
Second group: Sn
Sn: 0.50% or less Sn can be contained as necessary. When Sn is contained, it becomes Sn 3+ and dissolves, and corrosion can be suppressed by an inhibitor action in an acidic chloride solution. . In addition, by reducing Fe 3+ quickly and reducing the concentration of Fe 3+ as an oxidant, the corrosion promoting action of Fe 3+ is suppressed, so that the weather resistance in an environment with a large amount of incoming salt can be improved. it can. Furthermore, since Sn suppresses the anodic dissolution reaction of steel, the corrosion resistance can also be improved. However, if its content exceeds 0.50%, it causes embrittlement, so the Sn content is 0.50% or less. Preferably it is 0.20% or less. In addition, when obtaining these effects by Sn, it is desirable that the Sn content be 0.03% or more.
第３群：Ｃａ、Ｍｇ、ＲＥＭ
Ｃａ：０．００５％以下
Ｃａは、必要に応じて含有させることができる。Ｃａを含有させると、粒内フェライトの析出核となる酸化物、硫化物を生成して、また、硫化物の形態を制御して、低温靱性を向上させることができる。しかし、Ｃａ含有量が０．００５％を超えると大型介在物やクラスターを生成して鋼の清浄度を劣化させるので、Ｃａの含有量は０．００５％以下とする。好ましくは０．００３５％以下である。なお、Ｃａによるこの効果を得たい場合には、Ｃａの含有量は０．００１％以上とするのが望ましい。
Third group: Ca, Mg, REM
Ca: 0.005% or less Ca can be contained as necessary. When Ca is contained, oxides and sulfides that become precipitation nuclei of intragranular ferrite can be generated, and the form of sulfides can be controlled to improve low-temperature toughness. However, if the Ca content exceeds 0.005%, large inclusions and clusters are generated and the cleanliness of the steel is deteriorated, so the Ca content is set to 0.005% or less. Preferably it is 0.0035% or less. In addition, when acquiring this effect by Ca, it is desirable to make content of Ca 0.001% or more.
Ｍｇ：０．００５％以下
Ｍｇは、必要に応じて含有させることができる。Ｍｇを含有させると、粒内フェライトの析出核となる酸化物、硫化物を生成して、また、硫化物の形態を制御して、低温靱性を向上させることができる。しかし、Ｍｇ含有量が０．００５％を超えると大型介在物やクラスターを生成して鋼の清浄度を劣化させるので、Ｍｇの含有量は０．００５％以下とする。好ましくは０．００３５％以下である。なお、Ｍｇによるこの効果を得たい場合には、Ｍｇの含有量は０．００１％以上とするのが望ましい。
Mg: 0.005% or less Mg can be contained as necessary. When Mg is contained, oxides and sulfides that serve as precipitation nuclei for intragranular ferrite can be generated, and the form of the sulfides can be controlled to improve low-temperature toughness. However, if the Mg content exceeds 0.005%, large inclusions and clusters are generated and the cleanliness of the steel is deteriorated, so the Mg content is set to 0.005% or less. Preferably it is 0.0035% or less. In addition, when obtaining this effect by Mg, it is desirable that the content of Mg is 0.001% or more.
ＲＥＭ：０．００５％以下
ＲＥＭは、必要に応じて含有させることができる。ＲＥＭを含有させると、粒内フェライトの析出核となる酸化物、硫化物を生成して、また、硫化物の形態を制御して、低温靱性を向上させることができる。しかし、ＲＥＭ含有量が０．００５％を超えると大型介在物やクラスターを生成して鋼の清浄度を劣化させるので、ＲＥＭの含有量は０．００５％以下とする。好ましくは０．００３５％以下である。なお、ＲＥＭによるこの効果を得たい場合には、ＲＥＭの含有量は０．００１％以上とするのが望ましい。
REM: 0.005% or less REM can be contained as necessary. When REM is contained, oxides and sulfides that become precipitation nuclei of intragranular ferrite can be generated, and the form of sulfides can be controlled to improve low-temperature toughness. However, if the REM content exceeds 0.005%, large inclusions and clusters are generated to deteriorate the cleanliness of the steel, so the REM content is set to 0.005% or less. Preferably it is 0.0035% or less. In order to obtain this effect by REM, the content of REM is preferably 0.001% or more.
ここで、ＲＥＭとは、ランタノイドの１５元素にＹおよびＳｃを合わせた１７元素の総称であり、これらの元素のうちの１種または２種以上を含有させることができる。なお、ＲＥＭの含有量はこれらの元素の合計含有量を意味する。 Here, REM is a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanoid, and one or more of these elements can be contained. Note that the content of REM means the total content of these elements.
（Ｂ）鋼板の組織
（Ｂ−１）板厚中心部における結晶粒径２０μｍ以下のフェライト分率が６０％以上
上記成分系にて、板厚中心部まで充分な強度と全厚ＣＴＯＤ特性を両立するには、板厚中心部まで均一な微細フェライトを一定量以上分散させた上で、ε−Ｃｕ粒子を時効析出させる必要がある。結晶粒径２０μｍ以下の微細フェライトを体積％で６０％以上分散させることを実現すれば、板厚中心部にて降伏強度４００ＭＰａ以上と板厚４０ｍｍ以上の極厚材の全厚ＣＴＯＤ特性を両立させることが可能となる。
(B) Steel plate structure (B-1) Ferrite fraction with a crystal grain size of 20 μm or less at the center of the plate thickness is 60% or more In the above component system, both sufficient strength up to the center of the plate thickness and full thickness CTOD characteristics are achieved. In order to achieve this, it is necessary to disperse a certain amount of uniform fine ferrite up to the center of the plate thickness and then age precipitate the ε-Cu particles. If it is possible to disperse fine ferrite having a crystal grain size of 20 μm or less in volume% of 60% or more, the yield strength of 400 MPa or more and the total thickness CTOD characteristic of an extremely thick material having a plate thickness of 40 mm or more can be achieved at the center of the plate thickness It becomes possible.
板厚中心部における旧オーステナイト粒を規定する理由は、板厚の１／２の位置にある板厚中心部は板厚表裏面および板厚の１／４の位置に比べ特性の確保が困難なためであり、かつ全厚の特性が板厚の１／２の位置の特性に律速されるためである。 The reason for prescribing the prior austenite grains in the center of the plate thickness is that it is difficult to ensure the characteristics in the center of the plate thickness at the position of 1/2 of the plate thickness compared to the position of the plate thickness front and back and 1/4 of the plate thickness. This is because the characteristic of the total thickness is rate-limited to the characteristic at the position of ½ of the plate thickness.
（Ｂ−２）板厚中心部におけるＭ−Ａ組織が４．０％以下
本発明でいう「Ｍ−Ａ組織」は、いわゆる島状マルテンサイトであり、ベイナイトのラス状組織の間又は旧オーステナイト粒界に存在するマルテンサイトを指す。Ｍ−Ａ組織は脆性破壊の原因となるため、板厚中心部におけるＭ−Ａ組織を面積％で４．０％以下とする。このＭ−Ａ組織は少ないことが好ましい。
(B-2) The MA structure in the center portion of the plate thickness is 4.0% or less The “MA structure” referred to in the present invention is a so-called island martensite, which is between bainite lath structures or old austenite. This refers to martensite existing at grain boundaries. Since the MA structure causes brittle fracture, the MA structure in the center portion of the plate thickness is set to 4.0% or less in area%. It is preferable that this MA structure is small.
ここで、板厚中心部におけるＭ−Ａ組織を規定する理由は、中心偏析の影響により、特に板厚中心部でＭ−Ａが生成しやすく、かつ全厚のＣＴＯＤ特性は最脆化部の特性に支配されるためである。 Here, the reason for defining the MA structure in the center portion of the plate thickness is that, due to the influence of center segregation, MA tends to be generated particularly in the center portion of the plate thickness, and the CTOD characteristic of the full thickness is that of the most brittle portion. This is because it is dominated by the characteristics.
優れたＣＴＯＤ特性を得るには、このＭ−Ａ組織を４．０％以下に抑制する必要がある。このＭ−Ａ組織は少ない方が好ましい。ただし、本発明方法で製造した場合には、少なくとも０．５％のＭ−Ａ組織が形成されてしまう。 In order to obtain excellent CTOD characteristics, it is necessary to suppress this MA structure to 4.0% or less. It is preferable that this MA structure is small. However, when produced by the method of the present invention, at least 0.5% of the MA structure is formed.
なお、Ｍ−Ａ組織の割合は、透過型電子顕微鏡の倍率を１００００倍として合計１０視野観察し、各視野において求めた島状マルテンサイトが占める割合を算術平均して求めればよい。 Note that the ratio of the MA structure may be obtained by observing a total of 10 visual fields with a transmission electron microscope magnification of 10,000, and arithmetically averaging the proportions of island martensite obtained in each visual field.
（Ｂ−３）板厚中心部における介在物量がＪＩＳ Ｇ ０５５５における点算法にて０．０２０％以下
厚肉材の低温ＣＴＯＤ特性をさらに安定化するには、板厚中心部の介在物を低減することが有効である。特に、ＭｎＳを代表とするＡ系介在物を低減できれば、低温ＣＴＯＤ特性はさらに安定化する。具体的には、ＪＩＳ Ｇ ０５５５で規定された点算法で測定したときの非金属介在物量がｄ６０×４００（測定視野数が６０、倍率が４００倍）で０．０２０％以下とする。
(B-3) The amount of inclusions at the center of the plate thickness is 0.020% or less by the point calculation method in JIS G 0555. To further stabilize the low temperature CTOD characteristics of the thick material, the inclusions at the center of the plate thickness are reduced. It is effective to do. In particular, if the A-based inclusion represented by MnS can be reduced, the low-temperature CTOD characteristics are further stabilized. Specifically, the amount of non-metallic inclusions when measured by the point method defined in JIS G 0555 is d60 × 400 (the number of fields of view is 60, the magnification is 400 times) and is 0.020% or less.
（Ｂ−４）板厚中心部におけるＣが０．１２％以下
連続鋳造により製造した厚鋼板においては、板厚中心部での中心偏析による成分濃化部が生じやすく、これが硬化組織を形成すると破壊の起点になり、全厚ＣＴＯＤ特性が不安定となる。これを抑制するには、特に島状マルテンサイト生成に支配的なＣ量の偏析度規定が有効である。
(B-4) C in the thickness center portion is 0.12% or less In a thick steel plate manufactured by continuous casting, a component-enriched portion is likely to occur due to center segregation in the thickness center portion, which forms a hardened structure. It becomes the starting point of destruction, and the total thickness CTOD characteristic becomes unstable. In order to suppress this, the regulation of the segregation degree of the C amount that is dominant for the formation of island martensite is particularly effective.
板厚中心部のＣ量を測定するためには、レーザＩＣＰ分析法を用いることができる。レーザＩＣＰ分析法にて、板厚中心部を挟んだ８ｍｍ以上の範囲を分析し、次の式に基づいて、板厚中心部のＣ含有量を算出することができる。
板厚中心部のＣ量＝（製品分析値のＣ量）×（レーザＩＣＰ分析Ｃ量の平均値／レーザＩＣＰ分析Ｃ量の最大値）。
Laser ICP analysis can be used to measure the amount of C at the center of the plate thickness. A laser ICP analysis method analyzes a range of 8 mm or more with the plate thickness center portion interposed therebetween, and the C content in the plate thickness center portion can be calculated based on the following formula.
C amount at center of plate thickness = (C amount of product analysis value) × (average value of laser ICP analysis C amount / maximum value of laser ICP analysis C amount).
優れたＣＴＯＤ特性を得るには、この板厚中心部のＣ量を質量％で０．１２％以下とする必要がある。望ましくは０．１０％以下に抑制するとよい。 In order to obtain excellent CTOD characteristics, it is necessary that the C content at the central portion of the plate thickness is 0.12% or less by mass. Desirably, it may be suppressed to 0.10% or less.
以上、本発明の組織について示したが、本発明の鋼板のミクロ組織は主としてフェライトとベイナイトの混合組織でる。上記の（Ｂ）項で示すような組織を有する鋼板とするには、前記の（Ａ）項に記載の化学組成を有するスラブを用いて、たとえば、次の（Ｃ）項で述べる条件で鋼板を製造することができる。 Although the structure of the present invention has been described above, the microstructure of the steel sheet of the present invention is mainly a mixed structure of ferrite and bainite. In order to obtain a steel sheet having a structure as shown in the above section (B), a slab having the chemical composition described in the above section (A) is used, for example, under the conditions described in the following section (C). Can be manufactured.
（Ｃ）鋼板の製造条件
ここでは、加熱温度は炉内雰囲気温度、圧延終了温度は表層温度、水冷停止温度は板厚中心部の計算値とし、圧延、再加熱時の加熱、冷却速度についてはすべて板厚中心部の値とする。
(C) Steel plate production conditions Here, the heating temperature is the furnace atmosphere temperature, the rolling end temperature is the surface layer temperature, the water cooling stop temperature is the calculated value at the center of the plate thickness, and the heating and cooling rates during rolling and reheating are as follows. All values are at the center of the plate thickness.
（Ｃ−１）スラブの加熱温度
スラブの加熱温度は９００〜１１８０℃とするのがよい。スラブの加熱温度が９００℃未満では加熱時に均一なオーステナイト粒が得られない場合がある。一方、１１８０℃を超えて加熱するとオーステナイト粒が著しく大きくなって母材靱性が劣化する場合がある。したがって、鋼の加熱温度は９００〜１１８０℃とする。好ましい加熱温度下限は９５０℃であり、上限は１０５０℃である。
(C-1) Slab heating temperature The slab heating temperature is preferably 900 to 1180 ° C. If the heating temperature of the slab is less than 900 ° C., uniform austenite grains may not be obtained during heating. On the other hand, when heated above 1180 ° C., the austenite grains are remarkably enlarged and the base metal toughness may deteriorate. Therefore, the heating temperature of steel shall be 900-1180 degreeC. A preferable lower limit of the heating temperature is 950 ° C., and an upper limit is 1050 ° C.
なお、スラブの作製方法は問わないが、連続鋳造法により、溶鋼の温度を（凝固温度＋５０）℃以内に管理して作製することが好ましい。こうすることで、スラブ中の介在物や組成の中央偏析を小さくすることができる。 In addition, although the production method of a slab is not ask | required, it is preferable to manage and produce the temperature of molten steel within (solidification temperature +50) degreeC by a continuous casting method. By carrying out like this, the center segregation of the inclusion and composition in a slab can be made small.
（Ｃ−２）熱間圧延
鋼板の組織を適正化するために、未再結晶温度域で適正量の圧下（加工）を加えるのがよい。オーステナイトの再結晶温度域での圧下ではオーステナイト粒内に格子欠陥が蓄積され難く、圧延後に急冷しても組織の微細化が生じ難い。また、未再結晶温度域で圧下してもその累積圧下量が少ないと、オーステナイト粒内に蓄積される格子欠陥が少なくなって、圧延後に急冷しても組織の微細化が生じ難い。このため、７００〜８００℃の温度域で累積圧下率５０％以上の熱間圧延を行う。このとき、圧延は７００℃以上で終了させる。７００℃未満で圧下するとスラブ温度低下にともなうスラブ硬化により十分な圧下ができないだけでなく、圧延機に負荷がかかるため好ましくない。
(C-2) Hot rolling In order to optimize the structure of the steel sheet, an appropriate amount of reduction (processing) should be applied in the non-recrystallization temperature range. When the austenite is reduced in the recrystallization temperature range, lattice defects are unlikely to accumulate in the austenite grains, and even if it is rapidly cooled after rolling, it is difficult to make the structure finer. Further, if the cumulative reduction amount is small even if the reduction is performed in the non-recrystallization temperature range, the number of lattice defects accumulated in the austenite grains is small, and the structure is not easily refined even when rapidly cooled after rolling. For this reason, hot rolling with a cumulative reduction of 50% or more is performed in a temperature range of 700 to 800 ° C. At this time, the rolling is finished at 700 ° C. or higher. If the temperature is reduced below 700 ° C., not only is sufficient reduction not possible due to slab hardening accompanying a decrease in the slab temperature, but a load is applied to the rolling mill, such being undesirable.
（Ｃ−３）冷却工程
熱間圧延後は６５０℃以上の温度域から５００℃以下まで水冷を実施する。このとき、所望の組織を得るためには、板厚の１／４の位置において５℃／ｓｅｃ以上の平均冷却速度で鋼板を水冷する。冷却速度の上限は特に規定しないが、設備的に得られる上限値であってもよい。
(C-3) Cooling step After hot rolling, water cooling is performed from a temperature range of 650 ° C or higher to 500 ° C or lower. At this time, in order to obtain a desired structure, the steel sheet is water-cooled at an average cooling rate of 5 ° C./sec or more at a position of 1/4 of the sheet thickness. The upper limit of the cooling rate is not particularly defined, but may be an upper limit value obtained in terms of equipment.
熱間圧延後水冷された鋼は、その後再加熱し、５５０〜６７０℃の温度で時効処理を行う。時効処理によりＣｕを析出させ、鋼板を析出硬化させることができる。 The steel that has been water-cooled after hot rolling is then reheated and subjected to an aging treatment at a temperature of 550 to 670 ° C. Cu can be precipitated by aging treatment to precipitate and harden the steel sheet.
この際、時効処理の目的温度−１００℃までの昇温速度、および５００℃までの冷却速度については制御を行うことが望ましい。昇温／冷却速度の制御を行えば、Ｃｕ粒子の分散を均一化させることができる。十分なＣｕ粒子の分散均一化を図るためには、具体的には、再加熱の際の目標温度−１００℃までの平均昇温速度を１〜５０℃／分、目標温度での保持時間を１時間以上、冷却の際の５００℃までの平均冷却速度を１℃／分以上とするのが好ましい。 At this time, it is desirable to control the temperature increase rate up to the target temperature of aging treatment—100 ° C. and the cooling rate up to 500 ° C. If the temperature raising / cooling rate is controlled, the dispersion of Cu particles can be made uniform. In order to achieve sufficient dispersion and uniformity of Cu particles, specifically, the average temperature increase rate up to the target temperature of −100 ° C. during reheating is 1 to 50 ° C./min, and the holding time at the target temperature is The average cooling rate up to 500 ° C. during cooling for 1 hour or more is preferably 1 ° C./min or more.
表１に示す化学組成を有する鋼No.１〜４４及び鋼No.Ｘ１〜Ｘ１０の供試鋼から連続鋳造法にてスラブを作製した。表１における鋼No.１〜４４は化学組成が本発明で規定する範囲内にある例であり、そして、鋼No.Ｘ１〜Ｘ１０の供試鋼はいずれかの成分が本発明で規定する範囲から外れた例である。 Slabs were produced by continuous casting from steel Nos. 1 to 44 and steel Nos. X1 to X10 having chemical compositions shown in Table 1. Steel Nos. 1 to 44 in Table 1 are examples in which the chemical composition is within the range specified by the present invention, and the test steels of Steel Nos. X1 to X10 are ranges in which any component is specified by the present invention. This is an example that deviates from the above.
スラブは溶鋼の温度を過度に高くせず、溶鋼組成から決まる凝固温度に対し、その差が５０℃以内になるように管理をしながら、凝固直前の電磁攪拌および凝固時の圧下を行った。ただし、鋼No.３５および３６はこの管理を行わずに製造した。 The slab did not raise the temperature of the molten steel excessively, and the electromagnetic stirring just before the solidification and the reduction during the solidification were performed while controlling the difference between the solidification temperature determined from the molten steel composition to be within 50 ° C. However, steel Nos. 35 and 36 were produced without this management.
次いで、これらの鋼を通常の方法で厚さ１６０ｍｍの鋼片とした後、表２に記載の各種条件で熱間圧延、冷却して鋼板とした。なお、表２には記載していないが、すべての鋼について、再加熱の際に、再加熱温度−１００℃までの平均昇温速度を５℃／分、再加熱温度での保持時間を２時間、冷却の際の５００℃までの平均冷却速度を５℃／分として制御した。また、このとき、板厚は、海洋構造物用途を意識して、８０ｍｍ以上とした。 Subsequently, these steels were made into steel pieces having a thickness of 160 mm by a normal method, and then hot-rolled and cooled under various conditions shown in Table 2 to obtain steel plates. Although not shown in Table 2, for all the steels, the average heating rate up to the reheating temperature of −100 ° C. was 5 ° C./min and the holding time at the reheating temperature was 2 at the time of reheating. The average cooling rate to 500 ° C. during the time and cooling was controlled as 5 ° C./min. At this time, the plate thickness was set to 80 mm or more in consideration of the use of the marine structure.
このようにして得た各鋼板の板厚中心部から、ＪＩＳ４号引張試験片を圧延方向と平行な方向に採取し、母材の機械的性質（引張特性）を調査した。 JIS No. 4 tensile test specimens were collected in the direction parallel to the rolling direction from the center of the thickness of each steel plate thus obtained, and the mechanical properties (tensile properties) of the base material were investigated.
次に、ＢＳ７４４８規格に準拠し、全厚の３点曲げ試験片を圧延方向に直角の方向から試験片を採取して、ＣＴＯＤ試験を−４０℃で行った。 Next, in accordance with the BS 7448 standard, a three-point bending test piece of full thickness was taken from a direction perpendicular to the rolling direction, and a CTOD test was performed at -40 ° C.
また、溶接継ぎ手部は、ＢＳ７４４８に準拠し、Ｋ開先加工した鋼板突き合わせ部に１．０ｋＪ／ｍｍのＦＣＡＷ溶接を実施し、ＣＴＯＤ試験片の疲労ノッチがレ開先のストレート部側の溶接線となるよう加工し、ＣＴＯＤ試験を-４０℃にて実施した。 In addition, the weld joint is in accordance with BS7448, and 1.0 kJ / mm FCAW welding is performed on the K-grooved steel plate butt, and the fatigue notch of the CTOD test piece is the weld line on the straight part side of the groove The CTOD test was performed at -40 ° C.
さらに、各鋼板のうち一部の鋼板（試験番号No.８、１６、Ｘ３）については、得られた試験片をＳＡＥ（Society of Automotive Engineers）Ｊ２３３４試験により耐食性を評価した。ＳＡＥＪ２３３４試験は、湿潤（５０℃、１００％ＲＨ、６時間、塩分付着：０．５％ＮａＣｌ、０．１％ＣａＣｌ２、０．０７５％ＮａＨＣＯ３水溶液浸漬、０．２５時間）および乾燥（６０℃、５０％ＲＨ、１７．７５時間）を１サイクル（合計２４時間）とした加速試験であり、腐食形態が大気暴露試験に類似しているとされている（長野博夫、山下正人、内田仁著：環境材料学、共立出版（２００４）、ｐ．７４）。本試験は、飛来塩分量が１ｍｄｄを超えるような厳しい腐食環境を模擬する試験である。ＳＡＥＪ２３３４試験１２０サイクル終了後、各試験片の表面のさび層を除去し、板厚減少量を測定した。なお、「腐食減量」は、試験片の平均の板厚減少量であり、試験前後の重量減少と試験片の表面積を用いて算出したものである。 Further, for some of the steel plates (test numbers No. 8, 16, X3), the corrosion resistance of the obtained test pieces was evaluated by SAE (Society of Automotive Engineers) J2334 test. The SAEJ2334 test was wet (50 ° C., 100% RH, 6 hours, salt deposition: 0.5% NaCl, 0.1% CaCl 2 , 0.075% NaHCO 3 aqueous solution soaked, 0.25 hours) and dried (60 Accelerated test with 1 cycle (24 hours in total) at 50 ° C, 50% RH, said to be similar to atmospheric exposure test (Hiroo Nagano, Masato Yamashita, Hitoshi Uchida) Author: Environmental Materials Science, Kyoritsu Shuppan (2004), p.74). This test simulates a severe corrosive environment in which the amount of incoming salt exceeds 1 mdd. After 120 cycles of the SAEJ2334 test, the rust layer on the surface of each test piece was removed, and the thickness reduction amount was measured. The “corrosion loss” is an average reduction in the thickness of the test piece, and is calculated using the weight reduction before and after the test and the surface area of the test piece.
一方、各鋼板の組織は以下のように測定を行った。 On the other hand, the structure of each steel plate was measured as follows.
フェライトについては、光学顕微鏡を用いて観察によって得られた像を画像解析した。粒径を算出する場合には、短径と長径を測定し、その和の１／２から粒径を求めた。このようにして１００視野観察して求めた個々の粒子の粒径について、算術平均したものを「平均粒径」と規定した。また、フェライト分率は、１００視野観察分の面積に対するフェライトの面積割合を算出することによって求めた。また、島状マルテンサイト（Ｍ−Ａ）分率についても同様である。 As for ferrite, an image obtained by observation using an optical microscope was subjected to image analysis. When calculating the particle diameter, the short diameter and the long diameter were measured, and the particle diameter was determined from 1/2 of the sum. The arithmetic average of the particle diameters of the individual particles obtained by observing 100 visual fields in this manner was defined as “average particle diameter”. Moreover, the ferrite fraction was calculated | required by calculating the area ratio of the ferrite with respect to the area for 100 visual field observation. The same applies to the island martensite (MA) fraction.
板厚中心部におけるＣはレーザＩＣＰ分析にて、板厚中心部を挟んだ８ｍｍ以上の範囲を分析し、板厚中心部のＣ量＝（製品分析値のＣ量）×(レーザＩＣＰ分析Ｃ量の平均値／レーザＩＣＰ分析Ｃ量の最大値)として算出した。 C in the central part of the plate thickness is analyzed by laser ICP analysis over a range of 8 mm or more with the central part of the plate thickness interposed therebetween. C amount in the central portion of the plate thickness = (C amount of product analysis value) × (Laser ICP analysis C (Average value of amount / maximum value of laser ICP analysis C amount).
板厚中心部における介在物はＪＩＳ Ｇ ０５５５に準拠し、点算法による顕微鏡試験方法によって測定した。すなわち、試験片を圧延方向又は鍛錬方向に平行に、その中心線を通って切断採取し、その面を検鏡面とし、琢磨仕上げによって鏡面とした後、縦横２０本の格子線が入った接眼レンズを用い、ノーエッチングの状態で４００倍で、６０視野をランダムに観察し、介在物によって占めた格子点中心の数ｎを数える。そして、下記の式によって算出される清浄度ｄ（％）が非金属介在物の量である。
ｄ＝｛（ｎ／（ｐ×ｆ）｝×１００
ここで、ｄ：清浄度、ｐ：視野内の総格子点数、ｆ：視野数、ｎ：ｆ個の視野における介在物によって占められる格子点中心の数を表す。
Inclusions at the center of the plate thickness were measured by a microscopic test method using a point arithmetic method in accordance with JIS G 0555. That is, the specimen is cut and sampled through the center line parallel to the rolling direction or the forging direction, and the surface is made into a specular surface, and is made into a mirror surface by polishing, and then an ocular lens containing 20 vertical and horizontal grid lines , And 60 times are randomly observed at 400 times in a no-etching state, and the number n of lattice point centers occupied by inclusions is counted. The cleanliness d (%) calculated by the following formula is the amount of non-metallic inclusions.
d = {(n / (p × f)} × 100
Here, d: cleanliness, p: total number of lattice points in the visual field, f: number of visual fields, and n: the number of lattice point centers occupied by inclusions in the f visual fields.
表３に鋼板組織の観察結果を示す。 Table 3 shows the observation results of the steel sheet structure.
表４に機械的特性の試験結果を示す。ここで、鋼材の母材強度の目標はＹＳで４００ＭＰａ以上であり、母材および溶接部のＣＴＯＤ値の目標はいずれも０．３ｍｍ以上である。 Table 4 shows the test results of the mechanical characteristics. Here, the target of the base material strength of the steel material is 400 MPa or more in YS, and the target of the CTOD value of the base material and the welded part is 0.3 mm or more.
表４において、鋼の化学組成および組織等の要件が本発明で規定する範囲内にある試験No.１〜３４（本発明鋼）は、いずれも母材の強度とＣＴＯＤ値の目標をそれぞれ満足する。 In Table 4, Test Nos. 1 to 34 (steel of the present invention) in which the chemical composition and structure of the steel are within the range specified by the present invention all satisfy the targets of the strength of the base metal and the CTOD value. To do.
これに対して、試験No.３５は、鋼の化学組成は本発明で規定する範囲内にあるが、板厚中心部における介在物量が本発明鋼の規定より外れるため、母材のＣＴＯＤ値は目標に達していない。
試験No.３６は、鋼の化学組成は本発明で規定する範囲内にあるが、板厚中心部におけるＣ含有量が本発明鋼の規定より外れるため、母材のＣＴＯＤ値は目標に達していない。
On the other hand, in test No. 35, the chemical composition of the steel is within the range specified in the present invention, but the amount of inclusions at the center of the plate thickness deviates from the specification of the steel of the present invention, so the CTOD value of the base material is The goal has not been reached.
In test No. 36, the chemical composition of the steel is within the range specified by the present invention, but the C content at the center of the plate thickness is outside the specification of the steel of the present invention, so the CTOD value of the base material has reached the target. Absent.
試験No.３７〜４４は、鋼の化学組成は本発明で規定する範囲内にあるが、板厚中心部における結晶粒径２０μｍ以下のフェライト分率および島状マルテンサイト組織の面積率が本発明鋼の規定より外れるため、母材のＣＴＯＤ値は目標に達していない。さらに、試験No.３７および３８については、いずれもＹＳが目標に達していない。 Test Nos. 37 to 44 show that the chemical composition of steel is within the range specified in the present invention, but the ferrite fraction having a crystal grain size of 20 μm or less and the area ratio of the island martensite structure in the central part of the plate thickness are the present invention. The CTOD value of the base material has not reached the target because it deviates from the steel specification. Furthermore, YS does not reach the target for Test Nos. 37 and 38.
そして、試験No.Ｘ１〜Ｘ１０は、鋼の化学組成が本発明鋼の規定より外れるため、母材および溶接部のＣＴＯＤ値はいずれかまたは両方が目標に達していない。 In Test Nos. X1 to X10, the chemical composition of the steel deviates from that of the steel of the present invention. Therefore, either or both of the CTOD values of the base material and the welded portion do not reach the target.
なお、上述したとおり、一部の鋼板（鋼No.８、１６、Ｘ３）に関しては、ＳＡＥ（Society of Automotive Engineers）Ｊ２３３４試験により耐食性を評価した。その結果、試験No.８および１６については、Ｓｎを含有しているため、板厚減少量はともに０．１ｍｍ以下であり、高い耐食性を示した。しかし、試験No.Ｘ３については、Ｓｎが含有していないため、板厚減少量は０．８ｍｍとなり耐食性は低かった。 In addition, as above-mentioned, corrosion resistance was evaluated by SAE (Society of Automotive Engineers) J2334 test about some steel plates (steel No. 8, 16, X3). As a result, since test Nos. 8 and 16 contained Sn, the thickness reduction amount was 0.1 mm or less, indicating high corrosion resistance. However, for test No. X3, since Sn does not contain, the thickness reduction amount was 0.8 mm, and the corrosion resistance was low.
本発明の鋼板はＹＳが４００ＭＰａ以上、ＣＴＯＤ値も０．３ｍｍ以上であることから、氷海域の海洋構造物など溶接鋼構造物の素材として好適である。また、製造の観点からも、本発明の鋼板はオートテンパーを利用し焼戻しなしに製造することができるので、比較的容易且つ廉価に高強度の鋼板を得ることができる。 Since the steel sheet of the present invention has a YS of 400 MPa or more and a CTOD value of 0.3 mm or more, it is suitable as a material for welded steel structures such as marine structures in ice sea areas. Also, from the viewpoint of production, the steel sheet of the present invention can be produced without tempering using an autotemper, so that a high-strength steel sheet can be obtained relatively easily and inexpensively.
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